Abstract:

In shallowmarine carbonate successions of the Phanerozoic, evidence for shorter-lived subaerial exposure stages of ancient carbonate seafloors is commonly found in the presence of small-scale epi-karst solution pits in discontinuity surfaces. Under favorable conditions, these solution features are accompanied by soil features including calcretes or root traces, alveolar septal structures, petrographic evidence such as pendant cements, circumgranular cracks or pisoliths, bleaching and staining of carbonate rocks or circumstantial geochemical evidence. Perhaps more often, however, ancient carbonate successions lack undisputable evidence for meteoric karsting. Using a well exposed case example from the Aptian of Oman, we here document that the superficial visual field inspection of solution pits in discontinuity surfaces may lead to erroneous interpretations. Outcrops at
JabalMadar, a diapiric structure, allowfor an in-depth analysis of dissolution features in the regionally extensive top Shu'aiba discontinuity. The solution pits discussed here were investigated for their stratigraphic position, their orientation relative to bedding planes, diagenetic and petrographic features and their potential relation to extensional fractures related to the updoming of Jabal Madar. The mainmessage brought forward, is that under burial conditions, spatially localized, hypogenic carbonate leachingmay formfeatures that are easily mistaken for ancient meteoric epikarst. These features preferentially form in interstratal positions where fractured, massive carbonate rocks are capped by a major discontinuity surface overlain by non-fractured argillaceous sediments. Thus,while dissolution–reprecipitation processes in the burial phreatic realmare omni-directional in permeable carbonates, low-permeability, argillaceous fines are not, or to amuch lesser extent, prone to chemical corrosion.
As a consequence carbonate-aggressive burial fluids leach out pits at the carbonate–shale interface. These appear to protrude perpendicular as bowl-shaped depressions into the underlying limestones and are – in the case examples documented here – preferentially aligned along factures. These findings have significance for the interpretation of ancient epikarst features in shallow marine carbonate successions